This is the first demonstration that an LTR retrotransposon-derived Rtl1 plays an important role in fetal and neonatal muscle development, strongly suggesting the crucial involvement of RTL1 in several muscle symptoms observed in the Temple and Kagami-Ogata syndromes. The gene discussed is RTL1; the disease is multiple congenital anomalies due to 14q32.2 maternally expressed gene defect.